Added missing description of system.beta_version property.

src/sbbs3/js_system.c

@@ -451,6 +451,7 @@ static char* sys_prop_desc[] = {
 	,"public host name that uniquely identifies this system on the Internet (usually the same as <i>system.inetaddr</i>)"
 	,"Synchronet version number (e.g. '3.10')"
 	,"Synchronet revision letter (e.g. 'k')"
+	,"Synchronet alpha/beta designation (e.g. ' beta')"
 	,"Synchronet full version information (e.g. '3.10k Beta Debug')"
 	,"Synchronet version notice (includes version and platform)"
 	,"platform description (e.g. 'Win32', 'Linux', 'FreeBSD')"